The present invention relates generally to sewing machines of the type wherein the thread-carrying needle is laterally shifted to form sewn stitches in a zig-zag pattern and, more particularly, relates to a mechanism for timing rotation of the cooperating hook in synchronism with the lateral shifting movements of the needle.
Sewing machines capable of forming an ornamental chain of lock stitches in a laterally-shifting, so-called zig-zag pattern are well known and in widespread commercial use. In a typical commercial zig-zag sewing machine, the thread-carrying needle is driven from a main drive shaft of the machine through intermediary eccentric cam mechanisms to reciprocate lengthwise upwardly and downwardly through a throat plate in a stitching bed in the machine frame while the needle is laterally shifted leftwardly and rightwardly in alteration in timed relation to the lengthwise reciprocatory movements of the needle. A looptaker element having a peripheral hooked beak portion, commonly referred to as a hook, is rotatably driven beneath the throat plate in parallel relation to the lateral shifting movements of the needle by a secondary drive shaft driven from the main shaft in timed relation to the needle reciprocating and shifting movements. A thread-carrying bobbin is mounted stationarily alongside the looptaker. In operation, upon the completion of each downward stroke of the needle, a loop of the thread carried by the needle is formed as the needle begins its upward stroke, the timing of the rotation of the looptaker in relation to the needle being such that the hook of the looptaker seizes the loop and carries it around the bobbin to lock stitch the threads of the needle and bobbin together.
A long-standing and widely recognized problem in the operation of zig-zag sewing machines of the aforementioned type is that, since the looptaker is conventionally rotated at a constant speed about a fixed axis, the looptaker cannot be timed with respect to the needle to present the hook in an optimal disposition with respect to the needle at both of the laterally shifted leftward and rightward positions of the needle. Accordingly, it is conventional practice to coordinate the rotation of the looptaker with respect to the needle reciprocation as though the needle were being reciprocated in a non-shifting straight stitch position equidistant the leftward and rightward shifted needle positions for the zig-zag stitch. In this manner, the hook is equally out of optimal timed relationship with the needle at each of the needle's leftwardly and rightwardly shifted positions, whereby the hook prematurely takes the thread loop when the needle is shifted rightwardly in the direction towards the approaching hook and likewise is delayed in taking the loop when the needle is shifted leftwardly in the opposite direction away from the approaching hook. While the sewing machine is acceptably operable with this manner of timing of the rotating hook with respect to the reciprocating and shifting needle, missed stitches, broken needles and prematurely worn hooks still inevitably occur.
Various proposals have been made to regulate the depth of needle penetration through the throat plate with respect to the hook to compensate for the shifted dispositions of the needle to improve the timing of the needle with respect to the hook, as representatively disclosed in U.S. Pat. Nos. 1,159,523; 2,932,268; and 3,779,187. However, none of these arrangements are known to have met with any significant degree of commercial acceptance and success. In other types of zig-zag sewing machines, the looptaker or other looper device is arranged for lateral shifting in timed relation to the lateral shifting of the needle to achieve proper relative timing, as representatively disclosed in U.S. Pat. Nos. 2,690,723; 3,490,401; and 3,783,810. It is also known in another type of zig-zag sewing machine to provide a cam-controlled mechanism for intentionally producing a pattern of missed stitches by selectively advancing or retarding rotation of the looptaker out of timed relationship with respect to the needle, as disclosed in U.S. Pat. No. 3,804,042.